1. Field of the Invention
The present invention relates to the retaining device of an annular member, such as a circular lens.
2. Description of the Related Art
For example, a circular lens is fitted in the correspondingly circular opening of a lens holder and is retained at the peripheral edge by a lens retaining ring. The retaining ring is usually secured to the lens holder by set screws. However, in recent inexpensive lens holders made of molded plastic, no set screws can be used. Accordingly, the plastic lens holder has supporting projections (bosses) which are located along an imaginary circle coaxial to the circular lens to surround the circular opening. A plate like retaining ring (i.e., annular member) circular lens has a plurality of holes in which the supporting projections of the lens holder are fitted. The heads of the supporting projections which are fitted in the corresponding holes of the retaining ring are fused so as to be connected to the lens holder. By using the mechanism connecting the lens to the lens holder with the help of the retaining ring as mentioned above, manufacturing and assembling costs of the elements can be reduced.
In the connecting mechanism, the position and the diameter of the supporting projections of the lens holder must correspond to the position and diameter of the holes of the retaining ring as exactly as possible. However, it is very difficult to machine the lens holder and the retaining ring so as to precisely correspond the position and the diameter of the supporting projections of the lens holder to those of the holes of the retaining ring. If there is a slight deviation in position or disagreement in size, no assembly can be effected. To this end, the diameter of the holes of the retaining ring is made slightly larger than that of the supporting projections of the lens holder or the holes of the retaining ring are in the form of elongated holes. However, in this solution, the retaining ring tends to loosen (move relative to the lens holder) due to the slight clearance (play) between the supporting projections of the lens holder and the corresponding holes of the retaining ring. In such a situation where the retaining ring is secured to the lens holder with play by fusing the supporting projections as mentioned before, no precise positioning of the lens with respect to the lens holder or the retaining ring can be ensured. Furthermore, since the fusing conditions of the supporting projections depend on the size of the corresponding holes of the lens holder, no stable fusing can be realized.